A method for fabricating isolation device is disclosed. The method includes the steps of: providing a substrate; forming a shallow trench isolation (STI) in the substrate, the STI includes a first STI and a second STI, and the first STI surrounds a first device region and the second STI surrounds a second device region; forming a first doped region between and contact the first STI and the second STI; and forming a first gate structure on the first doped region, the first STI and the second STI.